The invention relates to a method for starting a multiphase, sensorless commutated, brushless electric motor.
A brushless electric motor (brushless direct current motor or BLDC motor) generally has a plurality of position sensors with whose aid the commutation times of the individual motor phases can be determined at all times. The position information of these sensors is already available at a standstill, i.e. before the first rotation has begun. This means that the BLDC motor having position sensors can always be started and commutated in strict accordance with the position information of the sensors.
However, a BLDC motor may also be operated without position sensors. Here, the BEMF (back electromotive force) voltage is used for determining the position. This BEMF voltage represents a voltage induced by the permanent magnetic rotor that, on rotation of the rotor (i.e. not at a standstill), occurs at the stator phases and that can be measured in the respective non-energized phase. Commutation then takes place subject to the zero crossings of the BEMF. On starting the motor and at low rotational speeds, it is not yet possible to reliably determine such a BEMF zero crossing. Thus for its start up, the motor has to be initially commutated without any position information. Consequently for start up, the BLDC motor is preferably controlled with specified commutation times like a stepping motor. Here, however, the efficiency is low.
Moreover, it is particularly difficult to determine or to ensure that the rotor does not omit a commutation step. In other words, that the rotor continues to turn with each commutation step and in particular does not block. This is particularly important for applications in which the BLDC motor drives a path-dependent position or actuating drive and an internal travel counter counts the commutation steps. If the motor blocks unrecognized, the internal travel counter shows a difference to the actual travel position of the actuator, which is critical for its operation and must thus be avoided at all costs.